Since the advent of modern man, energy storage has been of prime concern. It can be said that much of man's progress has been due to the ability to harvest and store energy for use at a later time, whether it be the harvesting and storage of crops from agriculture to be used later as a fuel supply for the human body or the damming of waterways in order to later use the stored potential energy. Within the last 300 years, however, the manipulation of electrical energy has allowed for the concentration and storage of large amounts of energy in much smaller volumes than previously possible.
The first electrical energy storage devices took the form of what are now known as capacitors. In early experiments, water in an electrically grounded glass jar was connected to an electrode attached to an electrical generator. When the delivery of electrical power was ceased, energy created by the voltage differential between the water and the ground was stored as space charges in the glass. Modern solid-state capacitors, where energy is stored in the form of an electrostatic field between a pair of conductive electrodes, are ubiquitous because of their relatively small size and ability to be readily manufactured. Such capacitors find particular use in the field of large-scale integration, such as in integrated circuits. In a capacitor, capacitance is directly proportional to the surface areas of the conductive layers and is inversely proportional to the separation distance between these layers. Capacitance also depends on the dielectric constant of the material separating the layers and the formula for its calculation is: C=∈A/d, where C is the capacitance, ∈ is the dielectric constant, A is the area of the electrodes, and d is the distance by which the electrodes are separated. The work (W) that creates the electric field by a voltage differential (V) at the electrodes, and hence the amount of energy stored is given by the formula: W=CV2/2.
Attempts have been made to increase the amount of energy stored in such capacitors by either increasing the surface area of the electrodes, decreasing the distance between the electrodes, increasing the dielectric constant of the dielectric layer, increasing the voltage, or any combination of these factors. In one example, U.S. Pat. No. 7,033,406 describes a coated ceramic powder in calcium magnesium aluminosilicate glass composite matrix with a high dielectric constant situated between nickel electrodes in a multilayer design. In a similar example, U.S. Pat. No. 7,488,536 uses a different high-dielectric constant matrix of coated ceramic powder in an organic polymer matrix. In both cases, energy storage is described as being due to both the dielectric constant of the matrix material and the high voltage difference between the electrodes. In yet another example, U.S. Pat. No. 7,672,113 describes a composite containing an epoxy-containing polymer that includes ferroelectric ceramic particles in the dielectric layer.
In another capacitor design, electrolytic capacitors place a thin layer of a dielectric material between a metal (commonly aluminum, tantalum, or niobium) and an ionic conduction liquid. Electrolytic capacitors have been widely used because they are small in size yet achieve a large capacity in comparison with other types of capacitors. Recent designs include wound-type solid electrolytic capacitors that employ as the electrolyte a polypyrrole-based, polythiophene-based, polyfuran-based, or polyaniline-based conductive polymer, or a TCNQ (7,7,8,8,-tetracyanoquinodimethane) complex salt. Electrolytic capacitors have historically been plagued by problems of current leakage and durability due to the formation of cracks in the dielectric over time, potentially leading to electrical breakdown. Recently, electrolytic capacitors have incorporated solid electrolyte layers and other solid support films in order to obviate these problems. See, for example, U.S. Pat. No. 7,885,054, which describes a wound solid electrolytic capacitor.
Although chemical batteries, a term adopted by Benjamin Franklin, derive their name from the parallel combination (battery) of several capacitors, they store their energy chemically and are charged and discharged by the movement of ions through an electrolyte, which separates the electrodes. It is a redox reaction at each electrode that powers the battery with anions migrating to the anode, where they are oxidized, and cations migrating to the cathode, where they are reduced. Common types of batteries include lead acid (SLA) batteries, nickel-cadmium (Ni—Cd) batteries, and lithium-ion (Li-Ion) batteries. SLA batteries can hold a charge for up to three years and are generally used to provide backup power during emergencies. Ni—Cd batteries provide a fast, even energy discharge and are most often used to power appliances and audio and video equipment. Li-Ion batteries have the highest energy storage capacity (generally twice the capacity of Ni—Cd batteries) and are used to power portable computers, cellular phones, and digital cameras to name a few. Although the stored energy density of a typical chemical battery is quite high when compared to that of a typical capacitor, its lifetime is limited due to irreversible chemical side reactions at the electrodes.
In an attempt to overcome this limitation, U.S. Patent Application Publication No. 20100255381 describes a capacitor/battery hybrid in which inclusions are embedded in a dielectric structure between two electrodes, where electrons tunnel through the dielectric between the electrodes and the inclusions, thereby increasing the charge storage density relative to a conventional capacitor. In addition, micro- or nano-structured electrodes are described, providing an enhanced interface area relative to the electrode geometrical area.
In another attempt to move away from electrochemical energy storage, U.S. Patent Application Publication No. 20090195961 describes a ‘quantum battery’ in which strong dipolar rutile-phase nanoparticles, such as TiO2 or SrTiO3, are included in an insulating polymer resin matrix in a capacitor-like design. The nanoparticles are described as becoming conductive in a semiconductor fashion by means of ‘virtual photon resonance,’ which is described as a new quantum physical effect. Resonance phenomena are also described as useful for the storage of energy in U.S. Pat. No. 7,837,813, where Stone-Wales defect pairs in a carbon nanostructure are employed. Stored energy is released by a chain reaction of phonons disrupting the defect pairs to generate more phonons until the lattice returns to its original hexagonal form and the energy is released in the form of lattice vibrations.
Although seemingly unrelated to the technology discussed above because they are designed to generate electrical energy instead of storing it, some solar cells have been designed to take advantage of the resonance energy transfer phenomena of surface plasmons. In one example, U.S. Patent Application Publication No. 20110023941 describes an energy conversion device that converts light to electricity by rectifying surface plasmon polaritons. In another example, U.S. Patent Application Publication No. 20100075145 describes metal-polymer hybrid nanomaterials in which the light emitted from n-conjugated organic polymers is coupled to the plasmons of a metal surface. This type of energy transfer can be used in light-emitting diodes, solar cells, and photosensors. In yet another example, U.S. Patent Application Publication No. 20100000598 describes a photovoltaic cell that includes metallic nanoparticles or nanostructures that absorb energy via the coupling of light to surface plasmons on the nanoparticles. The absorbed energy is then transformed into an electrical current. These disclosures illustrate the potentially very efficient process of resonance energy transfer. What is needed is an energy storage device that combines the efficiency of this phenomenon not just for the conversion or transmission of electromagnetic energy, but also for its storage.